The present invention is directed to a solventless method for making polycarbonates by a transesterification reaction between diaryl carbonate and aromatic bis hydroxy compound. More particularly, the present invention relates to the employment of a guanidine catalyst to facilitate the melt condensation between diphenyl carbonate and bisphenol A.
Methods for making polycarbonate by a solventless melt condensation reaction between diphenyl carbonate and bisphenol A are shown by Sakashita et al, U.S. Pat. No. 5,026,817. In one procedure, Sakashita et al uses reactants having a particular range of hydrolyzable chlorine. In another method, a mixed catalyst is used, such as a nitrogen containing basic compound and an aromatic hydroxy salt of an alkali metal or alkaline earth metal. Some of the preferred basic compounds are tetraalkylammonium hydroxides, such as tetramethylammonium hydroxide. It has been found, however, that during the polymerization reaction, the tetramethylammonium hydroxide decomposes to produce a variety of products which can contaminate the final polymer. A second catalyst, such as sodium hydroxide is often employed as a finishing catalyst.
A further method for making polycarbonates by melt transesterification of diphenyl carbonate with 2,2-bis(4-hydroxyphenyl)propane is shown by Yangsoo Kim et al, Industrial Engineering Chemistry Research, 1992, 31, 2118-2127. Kim et al employ a lithium hydroxide catalyst. The effects of reaction temperature and catalyst concentration and conversion of oligomer are analyzed.
Although alkali metal hydroxides have been found effective either as a finishing catalyst or as a condensation catalyst, these materials are carried into the final resin which can be adversely affected.
Basic catalysts which are sufficiently volatile, such as organic nitrogen compounds, for example organic amines, also have been evaluated as condensation catalysts. Although such organic compounds can be readily removed from the polycarbonate resin at the termination of the condensation reaction, color bodies are often generated. It would be desirable therefore, to provide organic compounds useful as catalysts for effecting the condensation between a diaryl carbonate and an aromatic bis hydroxy compound under melt conditions which are readily removable and which do not adversely affect the resulting polycarbonate resin.